1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method.
2. Description of the Related Art
Along with a recent wide spread of OA (office automation) equipment such as a personal computer and a word processor, various printing apparatuses are available for printing information output from such equipment on various printing media. Particularly, an inkjet printing apparatus has the advantages of causing less noise, running at a low cost, and having a compact size and structure relatively easily made to support color printing. For this reason, the inkjet printing apparatus is accepted by users for a wide variety of purposes.
Additionally, the volume per ink droplet used in an inkjet printing apparatus is made as fine as several pl (picoliters) or less so as to meet the recent requirement for higher definition printing. Furthermore, there has appeared an apparatus with a printing head which ejects ink droplets of 1.0 pl or less.
The volume of such a fine ink droplet is equal to that of a mist particle, so that it is difficult to control each ink droplet individually. To put it another way, from viewpoint of higher definition printing, it is preferable to apply ink droplets of, for example, 1.0 pl or less to desired positions on a printing medium with accuracy of μm order; however, it is difficult to achieve a desired accuracy because ink droplets thus ejected are influenced by the surrounding air flow.
This phenomenon is particularly a problem in printing at a higher speed. There is an example of an inkjet printing apparatus having an inkjet printing head (hereinafter, also simply referred to as a printing head) with arranged ejection openings. The inkjet printing apparatus performs printing on a printing medium, while moving the inkjet printing head in main scanning directions which are different from a direction of the ejection-opening arrangement. The main scanning of the printing head and the conveyance of the printing medium (sub scanning) are alternately repeated to perform printing. In such a configuration, it is necessary to move the printing head in the main scanning directions at a high speed in order to increase the printing speed. This printing head movement moves the air so strongly as to disturb the flying of the ejected ink droplets.
Moreover, the single ink droplet is divided into several droplets immediately after the ejection, and thus much finer ink droplets called satellites are formed. These finer ink droplets may either be applied to unintended positions, or may stay floating inside the space of the printing apparatus. Moreover, when ink droplets land on a printing medium, finer ink droplets bounce back from the surface of the printing medium. Such finer ink droplets and satellites (hereinafter, these are referred to as ink mists) stay floating in the air, and eventually are adhered to and accumulated inside the apparatus, resulting in various problems. Specifically, for example, the ink mists make the inside of the printing apparatus unclean, deteriorate proper operations of a movable portion of the printing apparatus by adhering thereto, cause various sensors to malfunction, and also adheres to the surface of a printing medium to make it unclean.
In order to deal with such problems, a method to control ink droplets has been proposed (for example, in Japanese Patent Laid-open No. 5-124187 (1993)) as follows. Specifically, an electric field is generated between a printing head and a printing medium, so that ejected ink droplets are attracted to the printing medium by an electrostatic force. Thereby, the ink droplets are applied to desired positions on the printing medium.
In the meanwhile, recently there arises a demand that an image captured by a digital camera be printed in as high quality as a silver halide photography. In order to satisfy such a demand, printing methods incorporating various ideas have been made. For example, in one of the methods, printing is performed without leaving any margin on end portions of a printing medium (hereinafter, referred to as “margin-less printing”).
In this respect, the present inventors have tested a technique, as described in Japanese Patent Laid-open No. 5-124187 (1993), to perform margin-less printing, and found a problem as follows.
FIG. 13 shows a schematic plan view for explaining a manner that the margin-less printing is performed on side end portions of a printing medium. A printing head 104 has multiple ejection openings arranged in a direction corresponding to a direction P in which a printing medium 105 is conveyed. The printing head 104 is capable of reciprocal movement (main scanning) in Q1 and Q2 directions which are perpendicular to the P direction. During the main scanning, ink is ejected from the ejection openings to perform printing. When the margin-less printing is performed on the side end portions of the printing medium, ink is ejected not only on an area within the width of the printing medium, but also on both areas of a predetermined amount ΔL outside the width. Thus, an area E indicated by a dash-dot line in FIG. 13 is an area where ink is ejected in total. Such setting of the area E is for preventing a margin from remaining on a side end portion of a printing medium even when the printing medium shifts in the Q1 or Q2 direction, due to, for example, an error in a mechanism for conveying printing media.
FIG. 14 shows a schematic side view for explaining a case where the margin-less printing is performed while an electric field is generated between a printing head and a printing medium. Reference numeral 107 denotes a platen which is disposed to a position facing a surface (ejection face) of the printing head provided with ejection openings. The platen 107 supports the printing medium 105 to flatten the printed surface of the printing medium 105. Reference numerals 120 and 121 denote members (ink absorbers) made of a material with a water-absorbing property so as to absorb ink which is ejected to an area out of a side edge of the printing medium 105 in the margin-less printing.
The platen 107 is formed of a conductive material. When the platen 107 is applied with, for example, a voltage of 700 V, the surface (surface supporting a printing medium) of the platen 107 is positively charged. Accordingly, polarization occurs in the printing medium being in contact with the platen 107. The supported surface (bottom surface) of the printing medium is negatively charged, while the opposite surface (top surface) facing to the printing head is positively charged.
Since the electric potential of the printing head 104 is zero, an electric field is generated between the printing head 104, and the top surface of the printing medium as well as the top surface of the platen 107. When ink droplets are ejected to the printing medium from the printing head 104, the ink droplets travel to and land on the printing medium 105. Although the liquid ink droplets ejected from the printing head 104 originally have a momentum in the ejection direction (downward direction in the drawing), the ink droplets travels toward the printing medium at an accelerated rate while being attracted to the positively charged top surface of the printing medium. Thus, ink droplets originally ejected to the area out of the printing medium do not land on the ink absorbers 120 and 121 where the ink droplets should reach, but are attracted to the positively charged printing medium. In this way, the ink droplets move in flying directions which are deflected as shown by circles A in the drawing, and land on the side end portions of the printing medium. As a result, the resultant image has a higher density on the side end portions of the printing medium than an image that should be obtained originally.
As described above, even though the electric field is generated between the printing head and the printing medium to improve an image quality, the image quality is consequently deteriorated, on the contrary, when the margin-less printing is performed.